The field of the invention is that of forming three-dimensional structures in integrated circuit processing, in particular DRAM cells and other structures involving vertical transistors.
State of the art Trench capacitor DRAMs use cells with vertical transistors in order to reduce space by stacking the transistor generally above the capacitor and to avoid problems with scaling the pass transistor.
Since the trench capacitor center electrode is located in the trench that also holds the transistor gate, the current path through the transistor body must extend transversely outside the trench and into the semiconductor substrate.
In the case of stacked capacitor cells with buried bitlines or in the case of buried wiring levels below vertical transistors, the current path must similarly extend transversely outside the trench carrying the buried bitline or wiring level.
Prior art methods of introducing dopants into the substrate have involved outdiffusing from a heavily doped layer of poly (the inner electrode) and heating the wafer to drive the dopant into the substrate. As dimensions shrink, the inevitable manufacturing process fluctuations result in a greater percentage variation in vertical height between the capacitor and the transistor. At the same time, reduction in ground rules requires closer lateral spacing between cells and prevents the use of an increased dopant outdiffusion to provide a reliable current path.
The invention relates to a method of making a three-dimensional electrical structure making contact between two circuit elements that are separated vertically and horizontally.
A feature of the invention is the diffusion of dopant from an aperture cut into a semiconductor substrate, thereby extending a conductive path laterally into the substrate.
Another feature of the invention is the opening of a diffusion window in the sidewall of a trench for entry of dopant to form a self-aligned conductive path.
Another feature of the invention is the use of a temporary spacer to provide the correct vertical location for a hardmask formed on the interior of a trench.
Yet another feature of the invention is the use of a temporary layer to define a diffusion window for diffusion of dopant into the substrate to form a self-aligned extension of the buried strap in a DRAM cell having a vertical transistor.
Yet another feature of the invention is the formation of a diffusion window for diffusion of a dopant into the (single crystal) substrate to form a self-aligned extension of a dopant in the substrate with vertical transistors and buried bitline or wiring level.